TW201318184A - Solar cell - Google Patents

Abstract

A solar cell including a reflector layer, plural semiconductor layers, and a cover layer is provided. The reflector layer has a first surface and a light scattering portion under the first surface. The light scattering portion has a relative dielectric constant variation along a lateral direction on the first surface, wherein the variation of relative dielectric constant is greater than about 3 in a circle formed by taking any point in the light scattering portion as a center with a radius of 1 μ m. The semiconductor layers are stacked on the first surface in sequence for receiving external light and producing electric energy. The cover layer is disposed on the semiconductor layers, wherein at least a first interface between the reflector layer and the bottom semiconductor layer or a second interface between the cover layer and the top semiconductor layer is substantially a smooth surface.

Description

Solar battery

This application relates to a solar cell, and more particularly to a silicon-based solar cell.

In recent years, environmental awareness has risen. In response to the shortage of petrochemical energy and the impact of the use of petrochemical energy on the environment, the research and development of alternative energy and renewable energy has become a hot topic, among which solar cells are the most popular. . Solar cells convert solar energy directly into electrical energy, and do not generate harmful substances such as carbon dioxide or nitride during power generation, and do not pollute the environment.

Tantalum-based solar cells are a common type of solar cell. The principle is to add a high-purity semiconductor substrate, such as germanium (Si), to some impurities to exhibit different properties to form a p-type semiconductor and an n-type semiconductor. Further, a p-type semiconductor is bonded to the n-type semiconductor to form a pn junction, and a built-in potential exists on the pn junction. This built-in potential drives the movable carrier in this area. When sunlight hits a pn-structured semiconductor, the energy provided by the photons may excite electrons in the semiconductor and create electron-hole pairs. The free electrons and holes that are excited are affected by the built-in potential, causing the holes to move toward the p-type semiconductor, while the free electrons move toward the n-type semiconductor. If the two electrodes are connected to p-type and n-type semiconductors respectively, and connected to an external circuit and a load, current will pass and be available.

However, there are still technical problems in the application of existing solar cells to be overcome. In particular, external light is not easily retained in the solar cell, so that the photoelectric conversion efficiency of the solar cell is limited to a considerable extent.

The present application provides a solar cell that achieves a light absorption effect by a change in a relative dielectric constant and a refractive index of a substance, thereby improving photoelectric conversion efficiency. The solar cell includes a reflective layer, a plurality of semiconductor layers, and a cap layer. The reflective layer has a first surface, and the reflective layer has a light scattering portion below the first surface. The light scattering portion has a change in relative dielectric constant in a lateral direction parallel to the first surface, and is in a circle centered at any point in the light scattering portion and in a circle having a lateral radiation radius of about 1 μm. The constant varies by more than about 3. The plurality of semiconductor layers are sequentially stacked on the first surface for absorbing external light energy to generate electrical energy. The cap layer is disposed on the semiconductor layer, wherein the reflective layer and the lowermost semiconductor layer have a first interface, and the cap layer and the uppermost semiconductor layer have a second interface, and the first interface and the second interface are At least one is substantially smooth.

The present application further provides a solar cell comprising a reflective layer, a plurality of semiconductor layers, and a cap layer. The reflective layer has a first surface, and the reflective layer has a light scattering portion below the first surface. The light scattering portion includes a plurality of patterns distributed within the reflective layer. The pitch of the center points of the adjacent two patterns is between about 200 nm and 325 nm. The distribution density of the plurality of patterns is between 3×10 ⁸ /cm 2 and 1×10 ⁹ /cm 2 , and the interval between adjacent patterns is between 60 nm and 130 nm. The plurality of semiconductor layers are sequentially stacked on the first surface for absorbing external light energy to generate electrical energy. The material mainly absorbing light in the semiconductor layers, for example, includes at least one of a single crystal germanium, a polycrystalline germanium, a microcrystalline germanium, or a nanocrystalline germanium. The cap layer is disposed on the semiconductor layer, wherein the reflective layer and the lowermost semiconductor layer have a first interface, and the cap layer and the uppermost semiconductor layer have a second interface, and the first interface and the second interface are At least one is substantially smooth.

The present application further provides a solar cell comprising a reflective layer, a plurality of semiconductor layers, and a cap layer. The reflective layer has a first surface, and the reflective layer has a light scattering portion below the first surface. The light scattering portion includes a plurality of patterns distributed within the reflective layer. The pitch of the center points of the adjacent two patterns is between about 100 and 200 nanometers. The distribution density of the plurality of patterns is between about 9×10 ⁸ /cm 2 and 4×10 ⁹ /cm 2 , and the interval between adjacent patterns is between 25 nm and 80 nm. . The plurality of semiconductor layers are sequentially stacked on the first surface for absorbing external light energy to generate electrical energy. Wherein, the material mainly absorbing light in the semiconductor layers is amorphous germanium, and the cap layer is disposed on the semiconductor layer, wherein the reflective layer and the lowermost semiconductor layer have a first interface, the cap layer and the uppermost semiconductor layer There is a second interface between the two, and at least one of the first interface and the second interface is substantially a smooth surface.

The above-described features and advantages of the present application will become more apparent and understood.

FIG. 1 illustrates a solar cell in accordance with an embodiment of the present application. The solar cell 100 of the present embodiment includes a reflective layer 110, a semiconductor laminate 120, and a cap layer 130. The reflective layer 110 has a first surface 112. The semiconductor stack 120 is disposed on the first surface 112, and the semiconductor stack 120 is composed of, for example, a plurality of semiconductor layers for absorbing external light energy to generate electrical energy. The cap layer 130 is disposed on the semiconductor stack 120. In order to improve the light absorption effect of the solar cell 100, the present embodiment fabricates a light scattering portion 160 having a relative dielectric constant change under the first surface 112 of the reflective layer 110, wherein the relative dielectric constant of the substance is The refractive index is positively correlated to provide a structure of light having a refractive index change.

In more detail, the refractive index of light in the medium has the following relationship with the relative dielectric constant of the medium: n ² = ε × μ, where n is the refractive index, ε is the relative dielectric constant of the medium, or the permittivity ( Relative permittivity), and μ is the relative permeability of the medium. Therefore, for convenience of explanation and operation, the refractive index change of the substance can be reflected by the modulation of the relative dielectric constant. In this embodiment or other embodiments below, the numerical definition of the relative dielectric constant corresponds to light having a wavelength between 400 nm and 1200 nm.

In the present embodiment, the light scattering portion 160 is distributed over the entire first surface 112, and the light scattering portion 160 has a change in relative dielectric constant in the lateral direction T parallel to the first surface 112. The range of this variation is that the relative dielectric constant changes by more than about 3 in a circle centered at any point in the light scattering portion 160 and having a radius of radiation of about 1 micrometer in the lateral direction T. That is, in the circular range of the radius of about 1 micrometer, the difference between the maximum value and the minimum value of the relative dielectric constant does not substantially exceed 3.

In other words, the light scattering portion 160 of the present embodiment can be used as a light absorption structure having a refractive index change, which contributes to an improvement in photoelectric conversion efficiency of a solar cell, and can be used in place of a known solar cell for providing suction. ) The rough side of the light effect. Therefore, the embodiment does not need to form a rough surface on any film layer of the solar cell 100, and the interface of any two adjacent film layers can be a smooth surface. For example, the first interface S1 between the reflective layer 110 and the semiconductor stack 120, or the second interface S2 between the cap layer 130 and the semiconductor stack 120 may be a smooth surface. Alternatively, even the interface between any two adjacent semiconductor layers in the semiconductor stack 120 can be a smooth surface. Here, the definition of the smooth surface means, for example, that the root mean square roughness (Root Mean Square roughness) is substantially less than 20 nm.

In this way, the surface recombination loss caused by the rough surface can be greatly reduced. In addition, since the present embodiment can reduce or even completely omit the rough surface in the solar cell 100, the defects of the subsequent film layer on the rough surface coverage can be avoided. Moreover, since there is no need to worry about the covering effect of the subsequent film layer, the thickness of the subsequent film layer can be reduced, which contributes to reducing the bulk recombination loss of the solar cell 100.

Based on the foregoing design, the present application can also adjust the range of the relative dielectric constant variation of the light scattering portion 160 as needed. For example, in other embodiments of the present application, the range of the relative dielectric constant change of the light scattering portion 160 may be further defined as: a relative dielectric distance in the range of substantially 1 micrometer in the light scattering portion 160, relative dielectric The constant change is substantially greater than three. That is, the difference between the maximum value and the minimum value of the relative dielectric constant does not substantially exceed 3 in the straight line range of about 1 micrometer in length.

Further, the relative dielectric constant change of the light scattering portion 160 in the lateral direction is substantially less than 120. That is, in the entire light scattering portion 160, the difference between the maximum value and the minimum value of the relative dielectric constant does not substantially exceed 120.

In addition, in view of the fact that the depth of the light scattering portion 160 affects the light scattering effect of the light on the light scattering portion 160, the present application can control the depth of the light scattering portion 160. For example, in the embodiment illustrated in FIG. 1, the distance D1 between the top of the light scattering portion 160 and the first surface 112 can be maintained within a certain depth, such as less than about 10 nanometers. That is, the top of the light scattering portion 160 may be flush with the first surface 112 or buried below the first surface 112 by substantially no more than 10 nanometers. Further, the distance D2 between the bottom of the light scattering portion 160 and the first surface may be substantially greater than 40 nm. That is, the bottom of the light scattering portion 160 may penetrate deeper than the first surface 112 to a distance substantially exceeding 40 nanometers, and may even penetrate the reflective layer 110.

In the embodiment shown in FIG. 1, the light scattering portion 160 is composed of, for example, a plurality of patterns 162 distributed in the reflective layer 110. FIG. 2 illustrates the distribution of the light scattering portion 160 within the reflective layer 110. Referring to FIGS. 1 and 2, the center point of the adjacent two patterns 162 has a pitch p, and the minimum gap (gap) of the adjacent two patterns 162 is g. The plurality of patterns 162 have a distribution density D on a plane within the reflective layer 110. Further, the pattern 162 of the present embodiment is preferably circular with a radius r. In other embodiments, the pattern 162 can also be a polygon, such as: a circle, a triangle, a square, a diamond, a sector, a trapezoid, or other suitable shape, or a hybrid pattern of at least two of the above. For the materials suitable for the semiconductor stack 12, their corresponding light trapping wavelengths or light absorbing wavelengths, and the design of the pattern 162, the present embodiment provides several reference schemes as shown in the following table. :

In the above table, the light scattering portion 160 is an example of a substantially circular pattern, but is not limited thereto. In the other patterns described in the other embodiments, the pattern design conditions of the light scattering portion 160 are based on the distribution density and spacing as the main considerations, and at least one of the different semiconductor laminates is the main absorption (trap) light. material. Of course, if the main absorption (trap) light material in the semiconductor stack comprises the above two different groups, in the light scattering portion matching table, the conditions of the absorption (trap) light material that can include two different groups are selected. For example, the distribution density is about 9×108 (pieces/cm 2 )~1×109 (pieces/cm 2 ) and the interval is about 60 nm to 80 nm; or which of the semiconductor laminates is mainly The absorbing (trapping) light material is selected in accordance with the pattern of the light scattering portion 160 required by the group described above.

On the other hand, in order to achieve a good light scattering effect and a light sinking effect, the present embodiment can be used in addition to a material having a positive relative dielectric constant or a material having a negative relative dielectric constant. The material composition of the light scattering portion 160 is mashed. For example, the light scattering portion may include a first material having a positive relative dielectric constant, such as yttrium oxide (SiO ₂ ), titanium oxide (TiO ₂ ), aluminum oxide (Al ₂ O ₃ ), gas (gas), or Vacuum voids, etc.; and a second material having a negative relative dielectric constant, such as a metal material such as silver (Ag) or aluminum (Al). In particular, if the first material having a positive relative dielectric constant has a high contrast with respect to other portions of the reflective layer or the semiconductor stack, it is more advantageous to improve the scattering effect. For example, the relative dielectric constant of the first material is, for example, 1 and the relative dielectric constant of the second material is less than -1. Further, the ratio of the total volume of the first material to the second material in the light scattering portion is, for example, about 50% to obtain a good contrast.

Provided as previously described in FIG. 2 is a light scattering portion 160 having a periodic pattern 162. Actually, the light scattering portion 160 to which the present application is applied is not limited thereto. For example, the light scattering portion 160 may be formed of a pattern of any size and arbitrarily arranged. On the other hand, the light scattering portion 160, which is composed of an arbitrarily sized, arbitrarily arranged pattern, may provide a light scattering effect for a specific wavelength of light over a periodic structure. In the case where the light-scattering portion is constituted by a non-periodic pattern, the aforementioned pitch, interval, radius, and the like mean an average value.

The architecture of the solar cell 100 proposed in the foregoing embodiments can be embodied as various types of solar cells. In particular, the resulting solar cells will vary with actual process variations. Several solar cells suitable for the design of the present application are listed below as possible possible examples.

3 illustrates a solar cell in accordance with another embodiment of the present invention. The solar cell 300 shown in FIG. 3 first provides a smooth transparent substrate (for example, a glass substrate) 310 as a cap layer, and a transparent conductive layer (for example, indium tin oxide, indium zinc oxide, or the like) is formed on the surface of the transparent substrate 310. Indium gallium oxide, aluminum oxide, or other suitable material, or a combination of at least two of the foregoing) 320. The surface of the transparent conductive layer 320 may have good flatness, for example, a peak-to-valley roughness of substantially no more than 100 nm. Next, a P-type doped amorphous germanium layer 330, an amorphous germanium (a-Si) intrinsic layer 340, and an N-type doped microcrystalline germanium (uc-) are sequentially formed on the surface of the transparent conductive layer 320. Si) layer 350. Thereafter, the reflective layer 370 and the microcrystalline germanium layer 350 are bonded, and the reflective layer 370 has the light scattering portion 360 as described in the foregoing embodiment to provide a light trapping effect. The intrinsic layer 340 of this embodiment acts as a light-sinking layer in the semiconductor stack, having a thickness of, for example, about 150 nm.

4 illustrates a solar cell in accordance with yet another embodiment of the present invention. The structure of the solar cell 400 shown in FIG. 4 is similar to that of the solar cell 300 of FIG. 3, the main difference being the process. The solar cell 400 is formed by using a reflective layer 470 having a light scattering portion 460 as described in the foregoing embodiment as a substrate, and sequentially forming an N-type doped microcrystalline layer 450 on the reflective layer 470, and having a thickness of about 150 nm. The amorphous germanium in essence layer 440, the P-type doped amorphous germanium layer 430, the transparent conductive layer 420, and an anti-reflection layer 410 as a cap layer.

FIG. 5 illustrates a solar cell in accordance with another embodiment of the present invention. The solar cell 500 shown in FIG. 5 is a tandem junction structure, and a transparent conductive layer 520 is formed on the transparent substrate 510, and a P-type doping is sequentially formed on the transparent substrate 510 as a cap layer. The amorphous germanium layer 530, the amorphous germanium in essence layer 540, the N-type doped microcrystalline germanium layer 550, the P-type doped amorphous germanium layer 560, the microcrystalline germanium in essence layer 570, and the N-type doped microcrystalline germanium layer 580, etc. . Thereafter, the reflective layer 590 and the microcrystalline germanium layer 580 are bonded, and the reflective layer 590 has the light scattering portion 592 as described in the foregoing embodiment to provide a light trapping effect. The height difference of the transparent conductive layer 520 of the present embodiment is substantially no more than 100 nm, and the amorphous germanium intrinsic layer 540 and the microcrystalline germanium intrinsic layer 570 are used as a light absorption layer in the semiconductor stack, wherein the amorphous germanium layer The thickness of the intrinsic layer 540 is, for example, about 300 nm, and the thickness of the microcrystalline intrinsic layer 570 is, for example, about 1500 nm.

6 illustrates a solar cell in accordance with yet another embodiment of the present invention. The structure of the solar cell 600 shown in FIG. 6 is similar to that of the solar cell 500 of FIG. 5, the main difference being the process. The solar cell 600 is formed by using the reflective layer 690 having the light scattering portion 692 as described in the foregoing embodiment as a substrate, and sequentially forming an N-type doped microcrystalline layer 680 and a microcrystalline germanium layer 670 on the reflective layer 690. a P-type doped amorphous germanium layer 660, an N-type doped microcrystalline germanium layer 650, an amorphous germanium intrinsic layer 640, a P-type doped amorphous germanium layer 630, a transparent conductive layer 620, and an anti-reflective layer as a cap layer 610.

FIG. 7 illustrates a solar cell in accordance with yet another embodiment of the present invention. The solar cell 700 shown in FIG. 7 is an N-type doped single crystal germanium substrate 710 as a base, and a P-type doped single crystal germanium layer 720 as an emitter is formed on the upper side of the single crystal germanium substrate 710 and The antireflection layer 730 is formed, and a reflective layer 740 having the light scattering portion 750 as described in the foregoing embodiment is formed on the lower side of the single crystal germanium substrate 710. Among them, the single crystal germanium substrate is used as a main layer of light-absorbing material in the semiconductor laminate. In addition, the solar cell 700 further includes a plurality of electrodes 760 disposed on the P-type doped single crystal germanium layer 720 and exposed to the outside of the anti-reflective layer 730. The thickness of the single crystal germanium substrate 710 of the present embodiment is, for example, about 100 μm.

Although the present application has been disclosed in the above embodiments, it is not intended to limit the present application, and any person skilled in the art can make some changes and refinements without departing from the spirit and scope of the present application. The scope of protection of this application is subject to the definition of the scope of the patent application.

100. . . Solar battery

110. . . Reflective layer

112. . . First surface

120. . . Semiconductor stack

130. . . Cover

160. . . Light scattering section

162. . . pattern

D1, D2. . . distance

S1. . . First interface

S2. . . Second interface

T. . . Landscape

r. . . radius

g. . . interval

p. . . Pitch

300. . . Solar battery

310. . . Transparent substrate

320. . . Transparent conductive layer

330. . . P-type doped amorphous germanium layer

340. . . Amorphous layer of amorphous germanium

350. . . N-type doped microcrystalline layer

360. . . Light scattering section

370. . . Reflective layer

300. . . Solar battery

410. . . Antireflection layer

420. . . Transparent conductive layer

430. . . P-type doped amorphous germanium layer

440. . . Amorphous layer of amorphous germanium

450. . . N-type doped microcrystalline layer

460. . . Light scattering section

470. . . Reflective layer

500. . . Solar battery

510. . . Transparent substrate

520. . . Transparent conductive layer

530. . . P-type doped amorphous germanium layer

540. . . Amorphous germanium

550. . . N-type doped microcrystalline layer

560. . . P-type doped amorphous germanium layer

570. . . Microcrystalline layer

580. . . N-type doped microcrystalline layer

590. . . Reflective layer

592. . . Light scattering section

600. . . Solar battery

610. . . Antireflection layer

620. . . Transparent conductive layer

630. . . P-type doped amorphous germanium layer

640. . . Amorphous germanium

650. . . N-type doped microcrystalline layer

660. . . P-type doped amorphous germanium layer

670. . . Microcrystalline layer

680. . . N-type doped microcrystalline layer

690. . . Reflective layer

692. . . Light scattering section

700. . . Solar battery

710. . . N-type doped single crystal germanium substrate

720. . . P-type doped single crystal germanium layer

730. . . Antireflection layer

740. . . Reflective layer

750. . . Light scattering section

FIG. 1 illustrates a solar cell in accordance with an embodiment of the present application.

FIG. 2 illustrates the distribution of the light scattering portion of FIG. 1 within the reflective layer.

3-7 illustrate various solar cells in accordance with various embodiments of the present invention.

100. . . Solar battery

110. . . Reflective layer

112. . . First surface

120. . . Semiconductor stack

130. . . Cover

160. . . Light scattering section

D1, D2. . . distance

S1. . . First interface

S2. . . Second interface

T. . . Landscape

Claims (33)

A solar cell comprising: a reflective layer having a first surface, and the reflective layer has a light scattering portion below the first surface, the light scattering portion having a relative lateral direction parallel to the first surface a change in dielectric constant, wherein a relative dielectric constant varies by more than about 3 in a circle centered at any point in the light scattering portion and having a radius of about 1 micrometer; the plurality of semiconductor layers are sequentially stacked The first surface is configured to absorb external light energy to generate electrical energy; and a cap layer is disposed on the semiconductor layers, wherein the reflective layer and the lowermost layer of the semiconductor layer have a first interface. The cover layer and the uppermost layer of the semiconductor layer have a second interface, and at least one of the first interface and the second interface is substantially smooth. The solar cell of claim 1, wherein the smooth surface has a Root Mean Square roughness of less than about 20 nm. The solar cell of claim 1, wherein the interfaces between the semiconductor layers are substantially smooth surfaces. The solar cell of claim 3, wherein the smooth surface has a root mean square roughness of less than about 20 nm. The solar cell according to claim 1, wherein the relative dielectric constant changes by more than about 3 in a range in which the lateral distance of the light scattering portion is about 1 μm. The solar cell of claim 1, wherein the light scattering portion has a relative dielectric constant change of less than about 120 in the lateral direction. The solar cell of claim 1, wherein a distance between a top of the light scattering portion and the first surface is less than about 10 nm. The solar cell of claim 7, wherein a distance between a bottom of the light scattering portion and the first surface is greater than about 40 nm. The solar cell of claim 1, wherein the light scattering portion comprises a first material and a second material, the relative dielectric constant of the first material is a positive value, and the relative dielectric of the second material The constant is a negative value. The solar cell of claim 9, wherein the first material has a relative dielectric constant close to one. The solar cell of claim 9, wherein the second material has a relative dielectric constant of less than about -1. The solar cell of claim 9, wherein the ratio of the volume of the first material to the second material in the light scattering portion is substantially about 50%. The solar cell of claim 1, wherein the semiconductor layers comprise: a first semiconductor layer, a first conductivity type, the first semiconductor layer is disposed on the reflective layer; and a second semiconductor layer The second conductive layer is disposed on the first semiconductor layer, and the first conductive type and the second conductive type are N-type and P-type. The solar cell of claim 13, wherein the semiconductor layers further comprise a first intrinsic layer disposed between the first semiconductor layer and the second semiconductor layer. The solar cell of claim 14, wherein the semiconductor layers further comprise: a third semiconductor layer, the first conductivity type, the third semiconductor layer being disposed on the reflective layer and the first semiconductor layer a fourth semiconductor layer having a second conductivity type disposed between the third semiconductor layer and the first semiconductor layer; and a second intrinsic layer disposed on the third semiconductor layer Between the fourth semiconductor layers. The solar cell of claim 13, further comprising a transparent conductive layer disposed between the cap layer and the second semiconductor layer. The solar cell of claim 13, further comprising a plurality of electrodes disposed on the second semiconductor layer, and the cap layer exposing the electrodes. The solar cell of claim 1, wherein the cover layer comprises a transparent substrate or an anti-reflection layer. The solar cell of claim 1, wherein the light scattering portion comprises a plurality of patterns distributed in the reflective layer. The solar cell according to claim 1, wherein the material mainly absorbing light in the semiconductor layers comprises at least one of a single crystal germanium, a polycrystalline germanium, a microcrystalline germanium, or a nanocrystalline germanium. The solar cell of claim 20, wherein a pitch of a center point of two adjacent patterns is between about 200 nm and 325 nm. The solar cell of claim 20, wherein the patterns have a distribution density of between about 3 x 10 ⁸ /cm 2 and 1 x 10 ⁹ /cm 2 . The solar cell of claim 20, wherein each of the patterns is a circular pattern, and the radius of the circular pattern is between about 50 nm and 125 nm. The solar cell of claim 20, wherein the gap between adjacent patterns is between about 60 nm and 130 nm. The solar cell according to claim 19, wherein the material mainly absorbing light in the semiconductor layers is amorphous germanium. The solar cell of claim 25, wherein the center point of the adjacent two patterns has a pitch of between about 100 and 200 nm. The solar cell of claim 25, wherein the patterns have a distribution density of between about 9×10 ⁸ /cm 2 and 4×10 ⁹ /cm 2 . The solar cell of claim 25, wherein each of the patterns is a circular pattern, and the radius of the circular pattern is between about 25 nm and 75 nm. The solar cell of claim 25, wherein the interval between adjacent two patterns is between about 25 nm and 80 nm. A solar cell comprising: a reflective layer having a first surface, and the reflective layer has a light scattering portion below the first surface, the light scattering portion comprising a plurality of patterns distributed in the reflective layer, The pitch of the center points of the adjacent two patterns is between 200 nm and 325 nm, and the distribution density of the patterns is between 3×10 ⁸ /cm 2 and 1×10 ⁹ /cm 2 . And the spacing between the adjacent two patterns is between about 60 nm and 130 nm; a plurality of semiconductor layers are sequentially stacked on the first surface for absorbing external light energy to generate electric energy, wherein The material mainly absorbing light in the semiconductor layer comprises at least one of a single crystal germanium, a polycrystalline germanium, a microcrystalline germanium, or a nanocrystalline germanium; and a cap layer disposed on the semiconductor layers, wherein the reflective layer and the lowermost layer The semiconductor layer has a first interface between the cover layer and the uppermost layer of the semiconductor layer, and at least one of the first interface and the second interface is substantially smooth. The solar cell of claim 30, wherein each of the patterns is a circular pattern, and the radius of the circular pattern is between about 50 nm and 125 nm. A solar cell comprising: a reflective layer having a first surface, and the reflective layer has a light scattering portion below the first surface, the light scattering portion comprising a plurality of patterns distributed in the reflective layer, The pitch of the center point of the adjacent two patterns is between 100 nm and 200 nm, and the distribution density of the patterns is between 9×10 ⁸ /cm 2 and 4×10 ⁹ /cm 2 . And the spacing between the two adjacent patterns is between about 25 nm and 80 nm; a plurality of semiconductor layers are sequentially stacked on the first surface for absorbing external light energy to generate electric energy, wherein a material that mainly absorbs light in the semiconductor layer is amorphous germanium; and a cap layer disposed on the semiconductor layers, wherein the reflective layer and the lowermost layer of the semiconductor layer have a first interface, the cap layer and the most A second interface is disposed between the semiconductor layers of the upper layer, and at least one of the first interface and the second interface is substantially a smooth surface. The solar cell of claim 32, wherein each of the patterns is a circular pattern, and the radius of the circular pattern is between about 25 nm and 75 nm.